Electronic structure of the water dimer cation
The spectroscopic signatures of proton transfer in the water dimer cation were investigated. The six lowest electronic state were characterized along the reaction coordinate using the equation-of-motion coupled-cluster method with single and double substitutions for ionized systems. The nature of the dimer states was explained in terms of the monomer states using a qualitative molecular orbital framework. We found that the proton transfer induces significant changes in the electronic spectrum, thus suggesting to employ time-resolved femtosecond spectroscopy to monitor the dynamics following ionization. The electronic spectra at vertical and proton-transferred configurations include both local excitations (features similar to those of the monomers) and charge-transfer bands. Ab initio calculations were used to test the performance of a self-interaction correction for density functional theory (DFT). It is found that the corrected DFT/BLYP method is capable of reproducing the correct ordering of the (H2O)2+ isomers, and thus may be employed in calculations of larger systems.